I would like to connect a 431 at one end of the wire and plug it into the idc terminals of an extension socket at the other.
My question is if the 431 is facing up with the contacts facing upwards and I push in the wires, left to right: blue, blue-white, orange, orange-white, which colour wires should I plug into 2 and 5 at the extension socket?
First of all, as I have said previously, I really don't think what you are doing is a good idea and certainly won't make any difference to possible interference. However, of you insist on doing it you only need to connect the blue pair to the outside left and outside right on the 431, leave the middle terminals unterminated. Then connect the blue pair to 2&5 at the extension. Polarity doesn't matter.